The invention relates to a system of optical writing/reading on a recording medium and to its application to recording disks. It is applicable for example to optical recordings such as digital optical disks used in data processing, compact disks, etc.
Random access to a video sequence requires the storage, on a removable disk, of very large quantities of information elements. The MPEG1 high-definition digital video transmission standard based on non-compressed audio-digital channels enables interactivity. However there is already a need, as of now, for improved images defined according to the MPEG2 (XS) standard, or even images from portable equipment enabling only a DCT (Discrete Cosine Transform) compression in the 25 Mbits/s range. Furthermore, in order to provide for a single recording standard, the duration of the program must be extended to three hours without any gap.
Electronic reporting has introduced a need for quality video (50 to 150 Mbits/s) for a period of 20 minutes. In the next 10 years, high definition will introduce a requirement that is four times greater.
The maximum format that can be tolerated today is 5" in indoor equipment and 3.5" in portable equipment. The recording densities required are therefore:
15 bits/.mu..sup.2 for 3 hours of MPEG2 recording on a 5" format; PA1 20 bits/.mu..sup.2 for 1 hour of large-scale consumer type DCT recording on a 3.5" format; PA1 12 bits/.mu..sup.2 for 20 minutes of ENG recording (75 Mbits/s) on a 5" format. PA1 multiplication by 25 of the performance characteristics of compact disks through the use of optical and electronic systems of greater efficiency; PA1 in the next 10 years, further multiplication by four of the performance characteristics through the use of frequency-doubled laser. PA1 at least one light source emitting three beams that focus at three distinct points on the recording medium; PA1 a diffraction device creating three orders of diffraction for each beam;
The description of the above products indicates a realistic path of development for optical recording technology:
The object of the invention is to achieve the recording/reading of 0.3 .mu. tracks at a density of 125 Kbpi by using a conventional laser.
The making of a spot capable of a resolution of 0.2 .mu. as understood by Shannon's theorem (2.5 cycles/.mu. at 50% of modulation) assumes the use of a digital aperture NA=2 for a laser with a wavelength 0.8 .mu.. However, the use of a more compact code (RLL=Run Length Limited or PRML=Partial Response Maximum Lightly codes), an equalization enabling the operating point to be pushed back -6 dB closer to the cut-off frequency and a red laser (0.65 .mu.) suggests that an aperture of about 1.2 is sufficient.
Conventionally, such an aperture may be obtained by the use of an immersion optical system. The index of the liquids may reach 2 in certain iodized (but toxic) compounds and is easily equal to 1.7 in non-toxic compounds. Although it is unrealistic, the use of such a system is not less feasible than the use of a microscope in a near field. It has also been proposed to carry out a dry immersion microscopy, i.e. by bringing the information medium close enough to the lens so that the evanescent wave coming from the lens is picked up by a layer with a high index covering the medium. It must be noted that magneto-optical disks are already covered with such a layer for the chemical protection and improvement of the contrast. The layers used at present give an index in the region of 2. It would not be unrealistic to use transparent layers in the red part of the spectrum having an index in the region of 4. The boundary potential of resolution of a red beam in such an environment is therefore about 0.05 .mu.m.
The invention relates to a system that can be used to keep a major barrier space between the objective and the medium, enabling the system to work even in the presence of dust.